The performance of current spacecraft propulsion systems can be limited, for example, by the low density specific impulse product of the propellants used. Furthermore, these propellants can be highly toxic (e.g., hydrazine, nitrogen tetroxide) and/or require special storage and handling (e.g., cryogens), which can complicate ground and in-space operations. Both monopropellant and bipropellant systems are currently in use for in-space propulsion. Typically, bipropellant systems (e.g., monomethyl hydrazine and nitrogen tetroxide) are heavier and more complex than monopropellant (e.g., hydrazine) systems.
New monopropellants, particularly a class of ionic liquids, are being developed to address the above problems with the current propellants. These ionic liquids can have high density, high specific impulse (hence high density - specific impulse product), and extremely low vapor pressure that can result in reduced toxicity. They can have a very low freezing point and decompose at high temperature without vaporization. This wide liquidus range makes ionic liquid propellants attractive for space use.
While ionic liquid propellants can offer superior propulsive performance, simplicity and lighter weight of a monopropellant system, and ease of storage and handling due to reduced toxicity, these propellants can be very difficult to ignite. This is true of most ionic propellants because of their extremely low vapor pressure. Unlike conventional propellants, the ionic propellants typically do not vaporize to any significant degree, which is required for ignition. Accordingly, conventional approaches currently being developed use a catalyst that is heated (typically, the 405 catalyst manufactured by Honeywell, Columbus, Ohio, now Aerojet Corp., Sacramento, Calif.) to initiate ignition. However, the heated catalytic ignition has some drawbacks. The catalyst needs to be heated to high temperatures (approximately 300° C.); there can be issues with the catalysts structural stability at these temperatures, and its effectiveness can diminish rather quickly (e.g., minutes) due to cumulative poisoning. The catalyst can also be subject to thermal shock with each ignition cycle, which contributes to a reduction in the catalyst's longevity. Thus, catalytic ignition can limit the operational lifetime of propulsion systems, lowers system reliability, and requires significant power for its operation.
For applications such as aircraft EPUs, which require only a few minutes of operation, the use of catalyst can be appropriate. One problem in an EPU system is that it can be difficult to get the catalyst heated to a desired temperature (e.g., a temperature sufficient for ignition) rapidly (approximately 250 ms).
Therefore, it can be desirable to ignite fuel rapidly. It can also be desirable to ignite fuel without the use of a catalyst.